I have been using the Pololu A4983 / A4988 stepper drivers for quite some time to drive my timelapse dolly’s stepper motor. I used them due to their relative low cost and their ease of use as well as the fact that they support 1/16 microstepping mode which when using a toothed belt with the timelapse dolly is perfect for tiny movements such as ramping.

A note on the microstepping, the steps are so small that it can even be used for focus stacking etc moving just 0.2 mm per step.

The only problem I have with these stepper drivers is that the chip is so tiny and even though rated up to 2A, without proper cooling they are realistically only able to deliver about 0.7 A before overheating and shutting down.

Since the design I use for the dolly is one the “U” configuration with the belt secured on either end of the rail and the motor driver on the sledge itself (along with the camera of course) a failure or thermal shutdown of the driver could be catastrophic in terms of cameras crashing to the ground.

I use BIG stepper motors and in order to drive them at their highest capacity I would ideally need to run the driver at 2A. If I set this without any cooling it will overheat in about 15 seconds and will go into thermal protection mode, this is understandable if you consider the size of the chip:

For those of you who have not seen it before:

Pololu stepper driver front and back. (Image credit pololu)

There have been a few incarnations of my heatsink endeavors but they all rely on a the premise of using epoxy to fix a larger heat conducting material onto the chip with some thermal paste in between the chip and the larger piece of metal and then fixing an even larger heatsink to that.

The first heatsinks I built were the following:

Rudimentary but a lot of cooling area.

As you can see it relied on a large piece of aluminum with some rods coming out of it, they were pressed into holed slightly smaller than the rods to ensure they would hold. I used a piece of U-Bar aluminium to with a hole in it with which I could attach the large heatsink.

This method worked relatively successfully but it made the whole thing rather fragile because if that ever caught on anything the chip would be ripped right off the board (fun fact, epoxy gets harder when heated). The current I could put through the chip with this heatsink was about 0.4V on the ref pin, meaning about 1A of current to the motors. This is acceptable but the transfer of heat was not optimal and I was relying on the epoxy, bonded to the components on the board to hold the whole thing in place. This was the Maximim of this setup.

The calculation for current by using the sense pin (ref pin) is ref * 2.5. 0.5 * 2.5 = 1.25A

The case in which the electronics (screen, button pad and arduino) were housed also contained the stepper driver and the subsequent heatsink, this meant that the box had to be quite big and should have some sort of air flow else everything could get rather hot.

A4988 with 5V regulator, Chip heatsink and match

In order to minimize this whole setup I purchased some chip coolers meant for high performance PC components such as north bridges etc. These copper heatsinks come with thermaly conductive tape to attach to the chips however their sheer size means the amount of heat they can dissipate is only about 1A worth (VRef of 0.4). After this the chip will go into thermal runaway and shut down. Pity that nothing is ever that simple. As a side note, this was one of the methods that I found while researching ways to heatsink a pololu.

Finally I changed the general design of the controller-dolly setup. Previously I had the following setup:

Power cable from battery to control unit

Camera trigger cable from unit to camera (on the dolly slider)

4 cable, high current cable from stepper unit inside control unit to stepper motor

This complexity meant when out in the field you had to worry about 3 cables, it was a pain.

I changed the design to move the stepper driver onto the sledge along with the optocoupler to fire the trigger, all I had to do was to change the 3 cables to 1 8 strand cable to send the various control signals from the arduino to the stepper driver and optocoupler circuit to drive the stepper as well as control the camera and then of course send back the 5V from the regulator to the arduino / screen and button pad.

This meant only 1 cable from arduino to dolly, and only one extra to the dolly sled in the form of the power cable.

With the added freedom of design I gained I was able to design a high power heatsink as space was no longer such an issue:

Pololu with small aluminum square tube epoxied to the chip. Chip is mounted on a breadboard which I designed for ease of connection.

Note the crocodile clip, that is the one measuring the vRef.

Closer View with thermal paste on top

Partly boxed, connecting cables soldered onto the board

Using thermal paste and a large aluminium bar I secured the bar tightly via two bolts onto the aluminium tubing.

After the “pain” I had when creating the wonderfully clear and smooth finish, the crazy sanding with 60 grit belt sander, then 200, then 600 water paper then 1200 water paper then hours of polishing it just was not feasible to make these things more often but since I had a shape I wanted I ordered some mold making silicone.

It comes in a tin with a catalyst which you obviously mix together, stir vigorously and pour around your item which you have encased in some sort of box.This produces a negative of the item which you can then easily use to replicate as you just pour in your resin and it does not stick to the mold. Brilliant.

I used some acrylic and a hot glue gun to quickly make this box.

Acrylic is perfect as it’s non porous meaning the silicone mold will easily pop right out when I remove the walls.

Box with device hotglued into it.

Unfortunately as can be seen in the first picture, when mixing the silicone many bubbles are introduced through the process of stirring so to remove them I could use a vacuum pump but I don’t have one so I found the “bombs away” method on Youtube. This method relies on the fact that the silicone is so viscous that if you let it stretch thin enough no bubbles can survive the thin end of the stretch. Maybe better explained in the below pictures.

Here my silicone in a cup with a hole made in the bottom covered with sticky tape.

Look at it go!

Here you see the silicone stretched extremely thin by gravity and the viscosity of the liquid

Make sure to land next to the object, not on it else you could introduce more bubbles, let the liquid flow over the object from the bottom of the mold box.

Nearly done.

The bottom of the mold box.

Here the completed thing.

Completed.

Now I will add another completed circuit and pour in some resin. Lets see.

As previously mentioned, I ordered a polish from the supplier of the Resin as well as another type which is a special plastic polish used to fix scratches in phone displays etc.

Both work very well although I think the plastic polish is even better for the glass like finish.

Here some pics of the results:

This is the first of the special projects, more of a prototype I have to admit.

Unpolished and polished sides

Top View of the finished product

I am a little bit disappointed with the result as some resin leaked into the jack while I was sealing it, of all things… I had to drill it out, this caused damage to it so now I have a semi-working plug, you live and learn, I now use clear epoxy to seal the plugs.

It’s not completely polished yet, I need a dremmel )

Pretty Clear!

New Version, this one is customer ready, well besides not being polished 🙂

I have been racking my brain going all oven town trying to find brasso or equivalent to polish the resin cast however whatever I tried the block ended up milky so I set up my cam with my 105mm Nikkor macro and extension tubes to see what is going on on the surface of the resin, why do you not shine!

Here is a closeup of the resin after using a polishing cream guaranteed to shine metal including chrome up to a luscious shine:

Yeah really shiny…

So next I used a polishing paste which was for silver cutlery and took another snap:

Better but what the hell

Next an even more mild abrasive, this one is made for “all types of materials”

Mildest of the shop bought abrasives

So after much polishing, even using a polishing “fur” on a drill press I thought about what else could be less abrasive than these commercial products…

Enter toothpaste.

The first one is of the teeth whitening variety, man oh man I have to stop using that on my teeth!

Lines are now vertical, I wanted to make sure I was actually doing something.

Next I moved to the kind with the red and blue stripes running through it:

You can see it’s starting to shine!

And some more elbow grease:

The native structure is starting to show

And more some, notice that I cannot get rid of those deep scratches made from the initial sanding/polishing

Firstly I am a little disappointed that I have not yet been able to buff the initial test cast to a clear glass like finish. It just won’t buff up even though it is so smooth now and I have tried various polishing compounds including a brasso equivalent (or maybe not) and a metal polish for chrome.

Orange tinge is due to the cardboard box leaking some colour

I sanded the piece with 600, 800 then 1200 water paper and subsequently started to polish it, unfortunately it is not reacting the way I had hoped. I even attached a polishing wheel to my drill press but even that only left the resin dull. I know it is crystal clear as before I sanded the top it was like water.

Another View

Here the trigger before sanding the cardboard mold away.

In it’s little conffin.

Coffin Removed!

With backlight, my work needs a little work 🙂

Bottom

And here some other projects I have on the go, please welcome the Nilsen Slider – Control box.

The resin set beautifully and I started to sand it down a bit, unfortunately it is a public holiday here in Swiss so I had to very quickly use the belt sander and I have subsequently started to sand it with some p800 and p1200 grit paper and I’ll try and polish it up to some kind of clear surface, unfortunately at the moment I only have some sub par cutlery polish, will try and get some brasso tomorrow.

I created the box, sealed the power and camera trigger jack, added wax around the pot, well actually I sealed the headphone jack with wax as well as the end was too small to successfully seal with sello-tape.

I hope the resin does not heat up too much and melt the wax, that would be catastrophic but it is what it is.

As a side note, when I used a bit of resin to seal the jacks form the back, the resin actually leaked into the jack and I had to drill it out, I trully thought it was ruined but luckily the jack still made contact with the cable, so for now, all is well.

The casket

Waxed outer I melted a candle with a heatgun and put it around the edges

Firstly my previous breadboard of spaghetti has been “steampunked” if that makes any sense, I wanted it to look completely authentic when I cast it in the resin, luckily it works!

Closeup view

And from afar

The resin I purchased from Swiss Composites is a “Wasser Klar” resin which directly translated means “Water Clear”. It has the consistency of thick sugar water, or maybe a little thicker and it mixes beautifully, it is a pleasure to work with, even the smell is quite mild.

I created the mold with cardboard and mixed some resin to close the backs of plugs and tried to close the pot, unfortunately I have no idea if the pot will leak and it will break everything, that would be quite terrible. The circuit took a surprising time to make 😦

I also mixed the resin with the suggested quantities, now we will see if it cures clear and with no cracks, it only raised about 4 degrees in temperature after setting into the mold.

Looking forward to tomorrow, lets see how it turns out.

*Update*

After 10 hours the resin barely started to jellify, it sure does take a while to cure.

Will check again this evening.

**Update 2**

After 28 or so hours the resin has seemingly hardened but is still slightly tacky.

It also show that it is not set as the temp is a degree higher than ambient.